Hydrostatic Design Stress

The design of AS/NZS 4130 pipes has been based on the static working pressure operating continuously at the maximum value for the entire lifetime of the pipeline. The value of maximum hoop stress used in the selection of the pipe wall thickness is known as the Hydrostatic Design Stress (HDS). This value is dependent upon the type of PE material being used and the pipe material service temperature. In AS/NZS 4131, materials are classified for long term strength by the designation Minimum Required Strength (MRS). The MRS is the value resulting from extrapolation of short and long term tests to a 50 year point at 20°C.

The Hydrostatic Design Stress (HDS) is obtained by application of a Design or Safety Factor (C) to the MRS. See the table below;

HDS =MRS/C

The specific value selected for the Design Factor depends on a number of variables, including the nature of the transmitted fluid, the location of the pipeline, and the risk of third party damage.

The wall thickness values for Series 1 pipes to AS/NZS 4130 were derived using a value of 1.25 for C, this being the minimum value applicable.

AS/NZS 4131 specifies MRS values of 8.0 MPa and 10.0 MPa for the grades designated as PE 80 and PE 100 respectively. The relationship between the S and MRS standard values in AS/NZS 4131 is as shown in Table 4.3. These standard values are polymer dependent and long term properties for each pipe grade material are established by long term testing to the requirements of ISO/DIS 9080 by the polymer producers. Individual PE grades may exhibit different characteristics and PE materials can be provided with enhanced specific properties. In these cases the advice of Vinidex engineers should be obtained.

Hydrostatic Design Stress and Minimum Required Strength Values

Material Designation

Minimum Required Strength

Hydrostatic Design Stress

(MRS) MPa

(HDS) MPa

PE 80

8

6.3

PE 100

10

8

Maximum Allowable Operating Pressure

MAOP =PN x 0.125/C

where:
MAOP is the maximum allowable operating pressure in MPa.
PN is the pipe classification in accordance with AS/NZS 4130.
C is the Design Factor.

For example, if the minimum value of C is chosen (C = 1.25), a PN 10 pipe will have a MAOP of 1.0 MPa at 20°C.

Where pipes are used to carry fluids other than water or in certain conditions where the consequences of failure are particularly significant, then another value of the Design Factor may need to be selected. The value selected will depend on both the nature of the fluid being carried and the location of the pipeline installation.

In the case of gas pipes in AS/NZS 4130, both Series 2 and Series 3, a Design Factor ranging between C = 2.0 and C = 4.0 applies depending on the specific installation conditions.

For specific installations, see the table below which is reproduced from AS/NZS 4130 and gives advisory information on selection of Design Factors

Where the Design Factor is varied, then the MAOP can be calculated as follows:

For Series 1 pipes:
For Series 2 pipes:
For Series 3 pipes:

The Design Factor is a combination of a number of sub factors (ƒx) which must be factored together to give the final value for C such that C is not less than the values for typical design factors tabulated above, and: